The present invention relates to a current measuring arrangement for providing at an output terminal an voltage proportional to a current to be measured, said current measuring device comprising a first field effect device and a second field effect device.
The present invention also relates to a voltage measuring device and a telephone terminal using a current measuring device according to the invention.
A current measuring device according to the preamble is known from the Unexamined Japanese Patent Application, Laid Open No. 59-221672.
Such current measuring devices find widespread use in applications in which a voltage should be provided representing a current flowing in a part of an electronic circuit. An example of such an application is a telephone terminal in which it is advantageous to make several transmission parameters dependent on the line current. Examples of such parameters are the gain of microphone and earpiece amplifiers. Also the control of the side tone balance can be made dependent on the DC line current to ensure a correct side tone suppression for all line lengths.
In the current measuring arrangement according to the above mentioned Japanese Patent Application, the current to be measured is applied to the source of a first field effect device. The drain of the first field effect device is connected to the source of a second field effect device. The drain of the second field effect device is connected to a power supply. The gates of the first and second field effect devices are connected to a corresponding reference voltage.
The voltage at the source of the second field effect device will change with the current to be measured because the second field effect device will adjust its gate voltage source to a value corresponding to the input current. This source voltage is applied to a source follower circuit which makes the output signal available at the output terminal.
Because the relation between the gate-source voltage and the drain current of a field effect device is non-linear, the relation between the current to be measured and the output voltage will also be non-linear. In general such a non-linear relationship is undesired in such a measuring arrangement.